Industrial processing facilities are often managed using process control systems. Example industrial processing facilities include manufacturing plants, chemical plants, crude oil refineries, and ore processing plants. Among other operations, process control systems typically manage the use of motors, valves, and other industrial equipment in the processing facilities. Radar level gauges (RLGs) are commonly used for measurements of the level of products such as process fluids, granular materials and other materials. A radar level gauge generally includes a transceiver for transmitting and receiving microwaves, a propagation device (e.g., an antenna) or a guided wave probe (i.e. transmission line suspended from top to bottom in the tank) arranged to direct microwaves and to couple returned microwaves affected by the product surface to the transceiver, timing circuitry adapted to control the transceiver and to determine the level based on a time relation between microwaves transmitted and received by the transceiver, and an interface arranged to receive power and to connect the radar level gauge externally thereof.
Fieldbus is the name of a family of digital industrial computer network protocols used for real-time distributed control of process control systems. A complex automated industrial system, such as a manufacturing assembly line, usually has a distributed control system that is organized as a hierarchy of controller systems. In this hierarchy, there is usually a Human Machine Interface (HMI) at the top level, where an operator can monitor or operate the system. This is typically linked to a middle layer of programmable logic controllers (PLC) via a non-time-critical communications system (e.g. Ethernet). At the bottom level of the control chain is the fieldbus that links the PLCs to the components that actually do the processing work, such as sensors (a type of transducer which senses a process variable which may include a transmitter referred to herein a process variable transmitter) and actuators, which may be collectively referred to as field devices, as well as processing equipment including tanks, burners and electric motors, console lights, switches, valves and contactors. The Highway Addressable Remote Transducer (HART) communications protocol is an early implementation of Fieldbus.
In addition to Fieldbus and HART, a set-point (PV, process variable) can be transmitted through current loop value provided from an analog output. The common industrial range for loop current HART devices is 4 mA-20 mA. HART communication overlays the traditional current loop (single value) method to provide additional measurement and diagnostic information.
The HART protocol makes use of the Bell 202 Frequency Shift Keying (FSK) standard to superimpose digital communication signals (using Frequency Shift Keying to encode digital information where a logical “1” is represented by a frequency of 1,200 Hz and a logical “0” is represented by a frequency of 2,200 Hz) on top of 4 to 20 mA DC analog signal at a low relative signal level. This enables two-way field communication to take place and makes it possible for additional information beyond the normal process variable to be communicated to/from a smart field instrument. The HART Protocol communicates at 1200 bps without interrupting the 4 to 20 mA signal and allows a host application (Hart master, such as a PLC) to get two or more digital updates per second from a smart field device. As the digital FSK signal is phase continuous, there is no interference with the 4 to 20 mA loop current analog signal. Additional device information is communicated using the digital signal. The digital signal in the case of a sensor generally includes device status, diagnostics, and additional measured or calculated values. Together, the two communication channels provide a low-cost and robust complete field communication solution that is generally easy to use and configure.
The HART Protocol permits all digital communication with field devices including process variable transmitters in either point-to-point or multidrop network configurations. In the multidrop mode the analog loop current in the HART enabled device is generally fixed at 4 mA in steady (quiescent) state, and it is possible to have more than one field device on one signal cable thus sharing a loop current coming from a common DC power supply. In this case each field device on the common line needs to have a unique HART communication address (or ID). The loop current analog output is not controlled anymore (other than for powering devices), its value is fixed, and PVs and data is provided solely through HART communication.
In the case of a sensor comprising a process variable transmitter, one example is a Guided Wave Radar (GWR) transmitter commonly used for level sensing in a tank, where the 4 to 20 mA current is output in the loop by the GWR transmitter device. Such GWR transmitters generally are two wire devices which need a DC power supply providing about 10 to 28 VDC and a minimum load HART termination resistance of about 250 Ohm within the loop to properly operate.